Post–Cardiac Surgery Management

Chapter 149 Post–cardiac Surgery Management



E. Christopher Orton, DVM, PhD, DACVS



KEY POINTS



• Impaired cardiac function in the form of inadequate cardiac output or elevated end-diastolic pressure, or both, is often present in patients that have undergone cardiac surgery.

• Overzealous administration of crystalloid fluids should be avoided in animals undergoing cardiac surgery.

• Inadequate cardiac output after cardiac surgery is suggested by resting blood lactate levels greater than 2.5 mmol/L or oxygen extraction ratio over 40%.

• Cardiac function in animals undergoing cardiac surgery should be supported with inotropic and vasoactive drugs.

• Ventricular tachycardia and atrial fibrillation are the most important arrhythmias occurring in animals after cardiac surgery.


INTRODUCTION


Cardiac function is impaired when cardiac output is inadequate despite adequate ventricular end-diastolic pressure or when adequate cardiac output is maintained at the expense of elevated ventricular end-diastolic pressure. Generally animals undergoing cardiac surgery have varying degrees of impaired cardiac function before surgery.1 Superimposed on this preexisting cardiac insufficiency are the detrimental effects that general anesthesia and thoracic surgery have on cardiopulmonary function. A positive element is that the cardiac repair should improve cardiac function. A guiding principle of cardiac surgery is that it should not be undertaken unless the prospect for improved cardiac function is significant.2 Nevertheless, these patients do require some special considerations for supportive care during the period immediately after surgery.



FLUID THERAPY


Fluid therapy after cardiac surgery often presents a dilemma. On the one hand, most animals benefit from some degree of volume loading during and after surgery to counteract the negative effects of anesthesia and surgery on cardiovascular function. This would include animals with preexisting cardiac insufficiency. On the other hand, patients often enter cardiac surgery with varying degrees of heart failure, and like any animal with heart failure, will not tolerate large loads of crystalloid fluids. The goal of fluid therapy in these patients is to maintain a vascular volume adequate to support cardiac function without worsening or precipitating congestive heart failure.


Assessments of vascular volume include body weight, central venous pressure (CVP), and pulmonary wedge pressure. The latter is ideal for animals with left-sided cardiac insufficiency but is not generally available. Thus body weight, CVP, and good clinical judgment are the best guides to fluid therapy. Preoperative body weight and CVP serve as good guidelines for the postoperative period if the animal was stable entering surgery. In general, a CVP of 4 to 8 cm H2O (3 to 7 mm Hg) is a reasonable therapeutic target. It is important to realize that animals with cardiac insufficiency are more likely to be volume overloaded than hypovolemic after cardiac surgery.


Overzealous fluid therapy is detrimental and inappropriate. As a general rule, animals should not receive higher than maintenance rates of crystalloid fluids after cardiac surgery. In fact, animals may be trying to excrete excess body water and sodium if the cardiac repair has substantially improved cardiac function. These animals may benefit from having little or no crystalloid fluid therapy. If blood volume is inadequate or there is ongoing loss of volume due to surgical bleeding, then volume replacement should take the form of colloid-type fluids such as fresh whole blood, plasma, washed red blood cells, or albumin. The hematocrit, plasma total solids, total protein, and colloid osmotic pressure serve as guides for the type of colloid fluid administered. Animals should be monitored closely for evidence of pulmonary or systemic congestion after cardiac surgery. If congestion develops, intravenous furosemide (2 mg/kg bolus or 0.25 mg/kg/hr constant rate infusion [CRI]) is indicated.



CARDIAC OUTPUT


Animals should be monitored for adequacy of cardiac output after cardiac surgery. Inadequate cardiac output is suggested by a resting blood lactate concentration greater than 2.5 mmol/L, mixed venous oxygen saturation less than 70%, or oxygen extraction ratio of over 40%.3 Low mean systemic arterial pressure suggests the possibility of, but does not confirm, inadequate cardiac output. Moderate hypotension (mean arterial pressure of 60 to 75 mm Hg) with evidence of generalized vasodilation (i.e., pink mucous membranes) and without evidence of inadequate cardiac output is well tolerated and does not generally require therapy to augment cardiac output. On the other hand, hypotension in a patient with generalized vasoconstriction (i.e., white mucous membranes, cold extremities) suggests that cardiac output may be inadequate.


Animals with evidence of inadequate cardiac output with or without concurrent hypotension should receive supportive therapy to improve cardiac output. Hypovolemia should be corrected, if present, but aggressive volume loading is not appropriate for animals with cardiac insufficiency. Therapeutic support of cardiac output takes the form of drugs or drug combinations with inotropic and vasomotor effects.


The choice of drugs is influenced by several factors, including the magnitude and chronicity of myocardial failure, mean arterial pressure, and renal function (Table 149-1).4


• Dobutamine, a synthetic analog with predominant β-agonist activity, is administered when inotropic support of the myocardium is desired. Stimulation of peripheral β2-receptors promotes vasodilation and can contribute to hypotension on occasion.

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Post–Cardiac Surgery Management

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